The internal numbering plan of a telecommunications switching system is the set of numbers (e.g., extensions, feature access codes) directly served by the switching system itself. It may be a subset of a network numbering plan of a telecommunications network of which the switching system is a part, or it may be a separate numbering plan from the network numbering plan.
Users who are directly served by the switching system (i.e., whose terminals are directly connected to the switching system) have direct access to the internal numbering plan; they can validly dial both the extension numbers and the feature access codes of the internal numbering plan. In contrast, switching systems typically provide only restricted external access to their internal numbering plans. That is, users who are not directly served by the switching system (i.e., whose terminals are connected to the switching system through other switching systems and through inter-system trunks in a telecommunications network) have direct access only to the extension numbers and have either no access or only indirect access to the feature access codes of the internal numbering plan.
Conventionally, external access to the internal numbering plan may be had in one of two ways. One way is to map the extension numbers of the internal numbering plan onto corresponding numbers of the network numbering plan. External users then dial network numbering plan numbers comprising the extension numbers prefixed by an office code that identifies the destination switching system to directly obtain call connections to corresponding internal extensions. But this scheme provides external access only to internal extension numbers and extensions and not to internal feature access codes and corresponding features. The other way is to have external users dial a single network numbering plan number to obtain a general call-connection to the destination switching system, obtain a second dial-tone from the accessed destination switching system in response as an indication that the switching system has been accessed, and thereafter dial the internal numbering plan extension numbers or feature access codes to obtain call connections to the corresponding extensions or feature modules just like internal users. But this scheme requires external users to first make themselves the equivalent of internal users, for purposes of accessing the internal numbering plan, by forcing them to first establish a connection to the switching system, and then to use this connection to input internal numbering plan numbers directly to the switching system as would internal users.
The reason for these restrictions on external access to the feature access codes of internal numbering plans is the failure of existing network numbering plans to provide for user selection of most features, and, in many cases, their failure to provide for the features at all. Consequently, these network numbering plans do not include user-invokable symbol sequences (e.g., user-dialable numbers) that correspond, within the network numbering plan itself, to the features or feature access codes of the internal numbering plans and which the users could invoke (dial) to access the features or feature access codes directly. In other words, the network numbering plans do not have feature access codes for internal numbering plan features.
A reason for entirely preventing external access to at least certain ones of the internal numbering plan feature access codes, such as long-distance network access codes, is to prevent abuses of the system, such as network toll fraud. But conventional switching system architectures make it very difficult, complex, and costly--if not impossible--to discriminate between different features and their access codes, so as to prevent external access to only certain features, or to limit external access thereto to authorized persons, yet permit external access to other, harmless, features such as leave-word calling. Consequently, designers of switching systems often "paint with a broad brush" and configure the systems to deny external access to all internal features and their access codes, even though optimally it is not desirable to do so.
Attempts have been made to overcome the limitations on external access to internal numbering plans. For example, U.S. Pat. No. 4,488,004 discloses an arrangement which allows direct external access to internal features of a private-branch exchange (PBX) in a PBX network, and thereby allows inter-PBX calls to appear to users as if they are served by a single large PBX. However, this arrangement requires the PBXs each to have an additional controller, and for these controllers to be interconnected by special dedicated control data links. This is a difficult, complex, and costly solution to the problem.